Bug 18752 - Automatic item modifications by age should allow 'blank' values

Bug 18752 - Automatic item modifications by age should allow 'blank' values

This patch modifies the automatic item modification by age template to
correct errors in form validation:

- Age in days should not be required
- A value should not be required in substitutions.

To test, apply the patch and clear your browser cache if necessary.

- Go to Tools -> Automatic item modification by age.
- Confirm that when editing or creating a rule the only required field
  is the substitutions field name.

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>